Nowadays, a user can easily obtain various information via a communication network with the spread of digital devices for wired or wireless communication such as a personal computer, a workstation, a PDA, a mobile phone, an IPTV receiver, etc. For an example, on the internet, the most popularized communication network, a user can access a web site with a digital terminal device such as a personal computer, and then utilize various contents, such as a document, a photo, a moving picture, a cartoon, a map, a blog, a mini-homepage, sound, music, a skin, an avatar, etc., from the web site. Usage of contents on the internet is being increased day by day.
However, since the information processing capability on the internet meets with a limit, there are reported many problems including retarded access to information, which is due to an overload.
In order to resolve the problems, a policy for an ISP (Internet Service Provider) to utilize dedicated cache servers and store caches therein is generally employed. Caching is helpful to reduce the traffic on the internet fundamentally and the time required to respond to a user's request.
FIG. 1 schematically represents the conventional system comprising dispersed cache servers.
With reference to FIG. 1, the entire system comprises a communication network 10, a plurality of dispersed cache servers 20, a plurality of user terminal devices 30 and a main server 40. Herein, a cache server 20 fetches frequently accessed information from the main server 40 via the communication network 10, and stores it on a storage device (not shown). Next, in case that a user terminal device 30 requests for information, a corresponding cache server 20 transfers the requested information directly to the user terminal device 30. Such implementation is generally called ‘caching’ and leads to improvement of the utilization efficiency of the communication network 10.
According to this caching technology, it is possible to provide information from a cache stored on a cache server 20 to a requesting user terminal device 30 without requiring it to directly access a main server 40. Therefore, the time to respond to a user's request may be remarkably shortened, and a load to a main server 40 and further to the entire communication network 10 may be decreased. Further, the communication network 10 may show robuster characteristics, even when it suffers from a local fault (e.g., a fault at a main server 40).
However, a cache server 20 as above cannot avoid a bottleneck phenomenon, in case that the number of users requesting for specific information grows and therefore the traffic by the users to access the information on the cache server 20 also increases. Thus, a method of dispersing cache servers 20 over areas as shown in FIG. 1 is generally preferred.
However, even with the dispersed cache servers, the following problems may be unresolved.
First, according to the conventional caching technology, physical and software obstacles may be frequently met. Generally, a cache server 20 and the storage device on the cache server 20 may be a source of a fault on the communication network 10 due to their own defects in hardware or software. Particularly, since the storage device for storing a cache itself is generally comprised of a mechanically structured hard disk, the storage device is required to be replaced with a new one due to its defects that may be generated when its unique threshold (e.g., a threshold for readings, writings or the like) has been reached. Especially in case that a plurality of cache servers 20 are dispersed at geographically distant locations, the cost and time to cure faults may also increase.
Further, according to the conventional caching technology, it is not easy to properly cope with an asymmetric routing environment. Most ISPs provide information to user terminal devices in a cache-based manner, and thus, generally apply a two-way transparency mode, where transparency means a feature that information is processed regardless of its storage location. However, according to the conventional caching technology, it is required for a normal service that a cache server 20 must receive the response by a main server 40 responding to a user's request in case that the user terminal device 30 originally issued the request toward the main server 40. However, under the asymmetric routing environment, it is difficult for a cache server 20 to receive all the responses from a main server 40. Therefore, the two-way transparency mode cannot be easily applied under the asymmetric routing environment, just following the conventional technology.
To resolve this problem, it may be supposed that a plurality of cache servers 20 be dispersed over smaller areas of the communication network 10, each of which is a symmetrical network. However, in this case, there may be a concern that each cache server 20 is located in too small an area, and thus, its cached information is also located in too small an area. This may lead to reduction of the traffic with each cache server 20 and degradation of the bandwidth usage.
Further, according to the conventional caching technology, each cache server 20 needs to have a high processing capability (e.g., a processing capability of no less than 10 Gbps). To meet this need, a great number of cache servers 20 and switches of a high capability are required. In other words, the total cost and/or the possibility of a fault being generated on the communication network 10 may increase.
Finally, according to the conventional caching technology, vulnerability to a fault at a central network may be problematic. Generally, since a central network composes a symmetrical routing environment, it is required to apply a switch into the central network and connect the switch to cache servers 20. Further, in case that a switch or a cache server 20 suffers from a fault, the entire communication network 10 may also suffer.
Therefore, there are urgent needs for advanced technologies for resolving the aforementioned problems and enabling efficient and stable caching.